Production of kh2po4, nacl and sodium phosphates



PRODUCTIGN F P0 NaCl AND SODIUM PHUSIHATES No Drawing. ApplicationFebruary 20, 1956 7 Serial N0. 566,359

4 Claims. (Cl. 23107) This invention relates to the production ofpotassium dihydrogen phosphate.

. For many years, the fertilizer industry has recognized the potentialadvantages of a concentrated fertilizer material rich in both potassiumand phosphate values. Extensive experimentation to produce eitherpotassium dihydrogen phosphate (KH POQ or potassium metaphosphate (K1 0has, as yet, failed to yield a commercially attractive process. Themetaphosphate contains 39.87% K 0 and 60.13% P 0 and is thereforeparticularly at: tractive as a highly concentrated (100%) plant food;the normal monopotassium phosphate (KH PO is only slightly lessconcentrated, for it contains 34.61% K 0 and 52.16% P 0 In addition ithas the desirable physical properties of being free-flowing andnon-hygroscopic even 'at high humidities. Furthermore the compound KH POis highly soluble in marked contrast to the insoluble KPO Many methodshave been proposed for the manufacture of the potassium phosphates,including a number based on the neutralization of phosphoric acid withpotassium carbonate or potassium hydroxide or potassium chloride. Boththe carbonate and hydroxide are disqualified as economicallyimpractical. Processes based on the reaction between potassium chlorideand phosphoric acid require an excess of concentrated phosphoric acidand for a substantial conversion to KH PO the highly corrosive reactionmixture must be heated to temperatures on the order of 250 C. Underthese conditions, the final reaction mass usually contains excessphosphoric acid which must be neutralized by potassium carbonate orpotassium hydroxide to avoid contamination of the potassium phosphateproduct, thus adding appreciably to the cost of the process. Furthermoreit appears that all the chlorine cannot be driven off from the reactionmixture without, at the same time, heating the phosphate to such a pointthat substantially all of it is completely dehydrated to produce themetaphosphate (KPO which, because of its insolubility is less desirablefor many fertilizer purposes.

. Because of the economic and technical difficulties present inprocesses involving interaction of phosphoric acid with KOH, K CO orKCl, others in the field have developed processes in which mixtures orcomplex compounds such as KH PO .H PO are involved. Still others haveendeavored to react KCl with NH H PO Instead of producing a purephosphate of potassium, the product in this case is inevitably a mixtureof potassium and ammonium compounds because of the fact that Nl-I H PO,and KH PO form a continuous series of mixed crystals.

This disadvantage does not exist in the case of the interaction ofmonosodium phosphate and potassium chloride. This was recognized byKlingbeil who, in German Patent 222,913, proposed the addition of KCl toa solution of NaH PO boiling down the solution until a condition ofsaturation with respect to NaCl is obtained and then cooling to efiectcrystallization of a substantial portion of the KH PO present. Thepatentee then Patent ice proposed to reheat the liquid remaining afterseparation of the KH PO to boil away an additional amount of water,thereby crystallizing out parts of the sodium chloride. Evaporation iscontinued until the saturation point for potassium phosphate at theboiling point is reached. Thereafter the solution is cooled to recoverpart of the potassium phosphate, which crystallizes out on cooling.Repetition of the steps of heating, boiling oli some of the water,crystallization of sodium chloride, cooling and crystallization ofpotassium dihydrogen phosphate, according to the patentee, carried theprocess to completion. The process has not achieved commercial success,even in Germany, because of the many expensive beatings and coolings,the corrosiveness of the solutions, and the latent possibility thatduring the crystallization of the sodium chloride some potassiumphosphate would also be thrown out of solution. Another such process isdisclosed in French Patent 706,548. The above disadvantages and variousother limitations in prior art processes based on the reacton:

are avoided by the practice of my invention.

that complete solution of both the NaH PO and KCl is achieved. Then thesolution is cooled to any convenient and economically justifiabletemperature to precipitate the greater portion of the potassiumdihydrogen phosphate (KH PO present, in crystalline form. After thecrystals are separated from the mother liquor, they are washed with icewater and then dried at 110 C. The mother liquor is treated withsufiicient soda ash and/or caustic to reach a pH level corresponding toNa i-IP0 or Na PO Because of the relatively low solubility of each ofthese salts at temperatures below 20 C., the phosphate values arereadily precipitated as the hydrated sodium salts and are recovered byconventional means. I then beat the remaining liquid which is nowsubstantially less corrosive than the unneutralized liquid. Theneutralized liquid contains any potassium not recovered as KH PO Heatingis continued to boil olf a portion of the water, and during this stepsodium chloride is thrown out of solution. When the equilibrium pointfor the system KCl-NaCl-H O is reached at the temperature ofevaporation, I filter off the sodium chloride. In order to recover thepotassium values, I employ the mother liquor in the preparation of theoriginal hot solution of NaH PO and KCl in a repetition of my process.

The following examples will serve to further illustrate the advantagesof my process.

Example I A six molar solution of NaH PO and KCl was pre-- pared bydissolving 720 parts by weight of NaH PQ; (anhydrous basis) and 448parts by weight of KCl in 1000 parts of water, heated to about C. Thesolution, after thorough mixing, was cooled to room temperature (20 C.)and about 71% of the KH PQ, was recovered in the form of crystals whichseparated from the cool solution. The solution was heated to about 80 C.and 144 parts by weight of caustic soda (anhydrous basis) were added tothe hot solution with stirring, and then cooled to precipitate out thesodium phosphate. Substantially all of the phosphate was recovered bycooling to about 5 C. (The solubility of trisodium phosphate l WhileIIhavedisclosed inrny V ing'the neutralizedliquidftothe-equilibriumipointgfor. the Y t 3 -K l+H20d O de zpr qpitateth max'irnum amount. of sodium chloridq-it will bel obv' to'others skilled inthe art thatthe' heating may-bejgis- .c ontinue'd shortof this point if a lowenrecoverypf sodithereof to :the remaining'solutiomithereby"precip at least one sodium phosphate from the grou'pco isaboutAigrams/liter at plflfCrand onlyligt ms/liter r at C.) r V V VThe alkaline filtrate was'then reheatedto therboiling point and;evaporation was continued untilthe; content. of

water wasfreduced to about 530.grams, sodium chloride being fthrown .outof solution "during ,thejevaporatron.

' f The'crystallinelsodium chloride wasseparatedfrom theremaining'jliquid in any convenient waysuch as filtra- The pH ofthe'liquid wasadjusted'to a pH of about 4.0 by' the. addition of asufiicient tion, decantation, etc.

amount of phosphoric acid solution, Whereuponthe liquid was returned tothe process to be usedin making up the next solution of NaH PO andKCL'Wiih due allowance p mime KCl content therein. r 'lnithis way anypotassium and phosphate values' not previously recovered, are recycledto minimize the losses of these constituents. i

' of a substantial portion "of phosphate by crystallization from asolution of potassium chloride and sodium dihydrog'en phosphate, theimprovethe potassium dihydrilgeni ment which comprises: recovering thephosphate values. 7

in the solution remaining aftercrystallization and'removal of asubstantial portion of the potassium dihydrcgen' phosphate froinithefsolutionioriginally formed. of. pot-assium chloride and sodiumdihydrogen phosphate by adding a base from the. group consisting ofcaustic .soda;and V sodaiashand mixtures thereofto-theremainingisolution; thereby precipitating at least one sodiumphosphatefrom the group consisting of Na i-IP0 and N 21 120 and theri recoveringthe insoluble phosphate from the remaining liquid. i 1 p 7 3. In aprocess tfor producing-potassium dihydrogen phosphate which includes ina first portion,'the recovery It'Wili. be appreciated that the alkalinefiltrate may W be neutralized before the evaporation stepf-withoutotherf wise altering the procedure described.

V V j Example 11 :EXample I was repeated excep tzthattheoriginals'o'lution was cooled below room temperature,lbygbeing re- Vfrigerat'ed to 5 C. V The recovered KH PO was thereby increased to about82% of the theoreticalyield, but in desiredKI- I PO V r r e -v method;heat urn chloride is permissible pr; desired; I'clairn;

f 1.7In a process-forproducing potassium tdihydrogen f 7 phosphate'which in cludesziu:afirst portion, therrecovery V orasubstantial.portiongof-the potassium .idihy'drogen phosphate bycrystallization fromla solution aspen-83mm chlorideand sodiurndihydrogen-phosphate,1.the: improve ment which comprises:separatelyi'recovering theTpotas- 'siurn and phosphate valuesiin ithe;solution -remaining 7 r ing the insoluble phosphate from' the remainafter crystallization and; removal 1 of alsub stantiallportion of the;putassiumidihydrogen. phosphate from the solution I V originally formedof potassium.;chloride1and 'sodium dfl d V hydrogen phosphate :by addingabasei'from :=th'e group .consistingof caustic s'oda iand soda ash and'mia'rtu'res' of Nag-IP0 andNa PO recovering the insoluble he PIOCBSSJ.V @177 Wei" "5 In a p c ss si inte im-t ns na a siu lphosphatewhichineludesjn a;first;; POI'HOIH, the recover the 'potas'sium-dihydrogenphosphate from j the. solution v originally iornied ofpotassiurnchlorideand sodium dihyi' drogenphosphfa't by-adding a-base from the'group conlii f lmt i l h qu d 'tdf b lj f j portion of t 1 .water contained-therein; 'therebyj precipitating sodium" chlorideysepar'ati-ng the solof a substantial portion of thepotassium diliydrogen phos- V phate by.crystallization from-asolution of potassium" chloride andsodiumdihydrogen'rphosphate; the improvement which 7 comprises:separately recovering the potas V siumand phosphate values in thesolution remaining after crystallization and removal :of a substantialportion of sistingof caustic soda'and' sodaash1and mixtures thereof tothe m fi r i fit i e y c tat Naz D'ei recoveringFthefinsolublephosphate: frornj the remaining sodiumjchloridegand recova er-ingthfpotaSsium-Valus 1 the liquid separatedfrom'the sodium-chloridepiecijaita e returning the saidliquidito the process for preparation ofan additional .solu'tion fof KCl and NaI- I PQ from whichKH PQ isto'be'recove red in a-repetition of the 'process. i

4. ln aprocess 'for producing potassium dihydrogen 7 phosphatewhichfincludes ina first portion, the recovery merit -which comp-rises;separately recovering the 7 potassiurn and phosphate values" "in thesolution. remaining of 'a :substantialportion of the potassiumdihydrogen'; 1 phosphate by crystallization irpm a.so1ution of'potassium V chloride and sodium dihydrogenphosphate, the improveaftercrystallization'andiremovaljofia substantial portion 7 tained ther ein,thereby precipitating sodiurn chloridacon Jtinuing the heating until"the equilibrium pointforjthe;

: solution ,of :K

system NaCl.*ICl-;H20.is reached forltheevaporation o i isfl empe ure;pressure, and'gextraneous iens present separating the solid sodiumchloride; and recdvciting. the potassiumivalues; inth'eliquid separatedfrom 1 the sodium chloride precipitatQbyfreturningthe said' iqui t e;qce 'slifqnpreparati pr an additional. 4 PQ from which 'mgpo gis Decl;1,1 936 'of'the potassium dihydrogenphosphateifromthe solution. i ly te d'Q rio assium l11 Iid a odium, ii: hydrogen phosphate by adding. causticsoda. ftofthelrle maining 5011.1iiOlLfthCIfibYflPIfiqmiiaiil'lgNagPQgrecover r 7 n f' 'q r heating the liquid toboil otha portionoflthewater cou L -MarL r2391? V r

1. IN A PROCESS FOR PRODUCING POTASSIUM DIHYDROGEN PHOSPHATE WHICHINCLUDES IN A FIRST PORTION, THE RECOVERY OF A SUBSTANTIAL PORTION OFTHE POTASSIUM DIHYDROGEN PHOSPHATE BY CRYSTALLIZATION FROM A SOLUTION OFPOTASSIUM CHLORIDE AND SODIUM DIHYDROGEN PHOSPHATE, THE IMPROVEMENTWHICH COMPRISES: SEPARATELY RECOVERLY THE POTASSIUM AND PHOSPHATE VALUESIN THE SOLUTION REMAINING AFTER CRYSTALLIZATION AND REMOVAL OF ASUBSTANTIAL PORTION OF THE POTASSIUM DIHYDROGEN PHOSPHATE FROM THESOLUTION ORIGINALLY FORMED OF POTASSIUM CHLORIDE AND SODIUM DIHYDROGENPHOSPHATE BY ADDING A BASE FROM THE GROUP CONSISTING OF CAUSTIC SODA ANDSODA ASH AND MIXTURES THEREOF TO THE REMAINING SOLUTION, THEREBYPRECIPITATING AT LEAST ONE SODIUM PHOSPHATE FROM THE GROUP CONSISTING OFNA2HPO4 AND NA3PO4, RECOVERING THE INSOLUBLE PHOSPHATE FROM THEREMAINING LIQUID, HEATING THE LIQUID TO BOIL OFF A PORTION OF THE WATERCONTAINED THEREIN, THEREBY PRECIPITATING SODIUM CHLORIDE, SEPARATING THESOLID SODIUM CHLORIDE; AND RECOVERING THE POTASSIUM VALUES IN THE LIQUIDSEPARATED FROM THE SODIUM CHLORIDE PRECIPITATE BY RETURNING THE SAIDLIQUID TO THE PROCESS FOR PREPARATION OF AN ADDITIONAL SOLUTION OF KC1AND NAH2PO4 FROM WHICH KH2PO4 IS TO BE RECOVERED IN A REPETITION OF THEPROCESS.